The present invention relates in general to imaging and detection equipment and methods, and in particular to a new and useful method and system of detecting the distance to and forming an image of objects in turbid and light attenuating mediums, utilizing laser light. Such turbid medium include the atmosphere on cloudy days for example and ocean water.
Under such conditions, a significant amount of light signals to be used for ranging and image formation, are strongly attenuated by dispersion along the transmission path and accompanied by disturbing scatter signals from the medium itself.
There are known distance measuring systems, from German patents and disclosures 26 34 627; 31 03 567; 32 19 452, for determining the time interval that has elapsed between a short reference laser pulse and a largely identical measuring pulse which is formed from the scattered light of the target object. It is sought in these references, to minimize the error in measuring of this time interval.
As is known, the range of a distance measurement with lasers in the atmosphere or a body of sea water, such as in laser-radar, lidar, and laser range finder methods, depends on the degree of turbidity of the transmitting medium much more than do microwave, or millimeter-radar, or solar methods. This is a disadvantage primarily in the military domain where hostile targets are to be detected with a scanning laser beam quickly and exactly, mostly under conditions of bad visibility. Considered in this connection are only the conventional systems based on the pulse travel time, where short pulses or pulse trains are directed from a laser transmitter in the direction of the target, and a receiver coaxial with or parallel to the axis of transmission is provided by which the reflected pulses are received, with this response being evaluated, in area scanning systems, as an image spot, and in systems with a rigid axis, as a distance, by a travel time measuring device.
The detection of laser echo signals upon scanning in turbid mediums is rendered difficult by two phenomena:
First, by the strong damping, or exponential attenuation, of the signal on its go and return travel through the medium, requiring mastering of high signal dynamics, and second, by a strong backscatter which is caused by the medium itself along the trace of the pulse to the target and which, while non-homogenously varying in time, produces signals that are frequently hard to distinguish from the echo signals coming from the target surface.